1. Field of the Invention
This invention relates generally to heat exchangers for centrifugal pumps and, more particularly, to heat exchanger thermal shields that enclose a heat exchanger that is located within the impeller chamber of a pump.
2. Description of the Related Art
A centrifugal pump generally includes a pump housing and an impeller assembly that attaches to a vertically-oriented pump shaft. The impeller assembly and pump shaft rotate within an impeller chamber in the pump housing. The impeller assembly includes an impeller and a cylindrical bearing journal which rotates relative to the pump housing walls. A cylindrical hydrostatic bearing surrounds the journal and maintains the impeller assembly in position. The pump housing includes passages that allow a flow of product fluid to circulate past the hydrostatic bearing to provide for the bearing function. See, e.g., U.S. Pat. No. 4,775,293 to Boster.
An internal heat exchanger typically is located in the impeller chamber. See, e.g., U.S. Pat. No. 4,005,747 to Ball. The heat exchanger includes concentric baffles that rotate with the pump shaft and includes concentric, cylindrical cup-like portions that fit between the baffles and that are stationary relative to the housing. The product fluid is circulated past annular clearances at the top and bottom of the hydrostatic bearing. At the bottom clearance, the fluid flows through a plurality of journal return holes into the impeller chamber toward the impeller, past the heat exchanger, along the pump shaft, and out the impeller chamber. An external cooling fluid circulates through the cup-like portions and therefore draws off heat in the product fluid. A generally cylindrical thermal shield covers the heat exchanger to attenuate the thermal gradients. The thermal shield is stationary relative to the rotating bearing journal.
After a centrifugal pump with a cylindrical internal heat exchanger has been in use for some time, a groove may be eroded into the inside surface of the bearing journal. This groove can weaken the journal, requiring repair or replacement of the impeller assembly, and, in certain environments, such as nuclear reactors, repair/replacement operation can be very costly.
It is believed that erosion of the bearing journal is caused by the confluence of the upper and lower bearing return flows at or near the heat exchanger's thermal shield. In some pumps, the journal return holes are located directly opposite the thermal shield, which typically has a flat bottom and a generally cylindrical, curved outer wall. Thus, lower bearing return flow from the journal return holes is directed to the corner of the heat exchanger and when combined with the upper bearing return flow causes a turbulent action resulting in secondary flows or vortices which are believed to be a major contributor to erosive patterns found on the inner surface of the bearing journal just above the heat exchanger corner.
From the foregoing discussion, it should be apparent that there is a need for a heat exchanger that reduces turbulence in the flow of fluid within the impeller chamber and minimizes or eliminates the flow from wearing a groove in the bearing journal. The present invention satisfies this need.